Companion planting is a technique gardeners use to enable plants to support each others growth and deter pest damage. Based on observations of historic gardening practices--the beneficial interactions among beans, corn and squash that comprise the Native American Three Sisters Garden--companion gardening emphasizes the relationships among plants. In 1978, author Louise Riotte, in "Carrots Love Tomatoes," reintroduced companion gardening to home gardeners, and it has become an important element in sustainable agriculture. Choosing companion plants for potatoes illustrates how companion gardening works.
Using companion planting can be as simple as following a chart. Before you plant, you plan, using a chart to put good-neighbor plants close to each other. This would result, in simplest terms, in placing potatoes close to bush beans, members of the cabbage family, and corn, perhaps with horseradish plants at the corners of the potato bed. Tomatoes, members of the cucurbit family, raspberries and sunflowers would be placed farther away.
A frequent consideration governing bad neighbors is membership in the same plant family. Planting tomatoes, potatoes, and eggplant close together, for example, results in what is called monoculture: reliance on a single crop. The most devastating example of monoculture in recent history was the Irish potato famine of the 1840s, caused by a viral blight. A comparatively minor version of a similar blight affected tomato crops in the American North during 2009. Planting family members close together means depletion of similar soil nutrients, which can weaken both soil and plants, making them vulnerable to disease.
The family variations in companion planting can contribute substantially to soil and plant health. Bush beans, for example, are known for fixing nitrogen in soil. Cabbages and other crops in the cabbage family use nutrients not as strongly needed by potatoes, keeping soil balance stable for companion crops.
Companion planting addresses pest problems in two ways. Bush beans, flax and horseradish, for example, deter potato bugs and blister beetles. Beans and potatoes join forces in mutual defense: Beans protect against Colorado potato beetles, while potatoes repel the Mexican bean beetle. The second strategy is called trap-cropping. Nightshade weed, a member of the same family as potatoes, attracts potato pests, which succumb to nightshade poisons. Unfortunately, nightshade is also poisonous to gardeners, who may not wish to let it grow. Eggplant possesses a similar allure for potato beetles. Riotte suggests planting eggplant as a border to potatoes, but you'll still need to remove the beetles from the eggplant.
Using Companion Planting
Arranging companion planting can challenge the home gardener as much as the seating plan for a large party. You may be tempted to hire a larger hall, or plow up the whole back yard, to make room for all preferences. Several strategies can help. One is the use of containers--some gardeners recommend that horseradish planted as a potato companion be restricted to containers, to contain its natural exuberance. Large containers enable plants to keep soil nutrients to themselves; new potato bags--and other crop bags--advertise that they can enhance and increase crop yield. Raised beds are another strategy, creating micro-growing climates for companions and keeping bad neighbors separated. In a small garden, one raised bed for potatoes, cabbage, and other favorable companions and a separate bed or containers for tomatoes, provide at least some separation.